The present invention concerns a method for controlling a common rail injection system for internal combustion engines, in particular for turbochargeable diesel engines.
The requirements for successful application of a diesel injection system include varying the variation parameters, possible with a given set of injection equipment, in such a way that optimum results in terms of exhaust emissions, fuel consumption, and noise emission can be achieved.
In addition to variations in the components used, for example nozzles, injectors, etc., variation parameters in the case of common rail systems may include injection onset, rail pressure, and possibly additional injection events.
Non-steady-state processes, such as, for example, in the event of a sudden elevation in the load on a turbochargeable diesel engine, may result in a sudden boost pressure deficit that can result in a sudden sharp elevation in particulate emission. A boost pressure deficit of this kind may be counteracted by boost pressure-dependent limiting of the maximum injection volume. As a result of this action, however, the time required for full engine torque to be made available becomes longer.
With turbochargeable engines in particular, in contrast to conventional naturally aspirated engines, the matter of whether the engine is operating at a steady-state load (i.e., constant injection volume) or non-steady-state load is believed to be important. The reason is that in turbochargeable engines, the exhaust flow drives a turbine which in turns acts upon a compressor that forces fresh air into the combustion chamber. When more fuel is injected into the combustion chamber, the higher energy in the exhaust gas results in a higher boost pressure. The boost pressure therefore depends on the injection volume. A steady-state operating point (constant load and engine speed) therefore results in a balanced state, i.e. the turbocharger rotates at a constant speed. In non-steady-state load conditions, this balanced state does not exist. This may be explained by way of an example: Let us assume that an engine is running at an engine speed N1, associated with which are a maximum permissible injection volume Me1, a boost pressure p1, and a corresponding air volume ML1 that is delivered by the turbocharger. If, however, the engine is transferred in non-steady-state fashion into this operating condition, the injection volume Me is abruptly increased from a smaller volume to the value Me1. The turbocharger is at the same time delivering a boost pressure P2 that is less than p1. The result, in the case of a non-steady-state load increase, is that the boost pressure and therefore combustion air are insufficient, and combustion is degraded. In this situation the engine would emit a smoke pulse during the non-steady-state load elevation. This smoke pulse not only is visible, but is also believed to have a negative effect on the particulate result in a transient emissions test.
To prevent or at least reduce this kind of smoke pulse, in the context of a non-steady-state load elevation, the injection volume has been limited as a function of the engine speed and the boost pressure (a so-called xe2x80x9csmoke characteristicsxe2x80x9d diagram).
In U.S. Pat. No. 4,841,936 is discussed a fuel injection control apparatus of an internal combustion engine in which a number of fuel injectors are associated with a pressurized fuel accumulating chamber. Also provided are drivers for controlling the fuel to be delivered, as well as controllers for adapting the pressure in the pressure chamber to a predetermined value.
In European Patent 0 812 981 is discussed a method for controlling injection while a turbochargeable diesel engine is in a non-steady state, and in which injection pressure is varied with increasing engine load, although the pressure increase performed upon detection of a non-steady state is less than in the case of a corresponding steady state.
An object of an exemplary embodiment of the present invention is to provide a control system of a common rail injection system with which the total particulate emissions can be reduced, in a relatively simple fashion.
In particular, it is believed that an elevated injection pressure in the presence of a non-steady-state engine load condition made available by the exemplary method according to the present invention yields substantially better atomization and, as a result thereof, a substantially smaller smoke pulse. It is also believed that the elevated rail pressure is no longer necessary once the non-steady-state engine load condition has ended, since sufficient boost pressure is once again available, so the rail pressure can be lowered again to the normal level (as a function solely of the injection volume).
Advantageously, in the presence of a non-steady-state engine load the rail pressure is elevated by a constant amount in each case, as compared to the rail pressure in the presence of a steady-state engine load, with identical injection volumes. Adding a constant value to a steady-state characteristic curve in this manner can be accomplished easily and without complexity in terms of control engineering.
An exemplary differential rail pressure amount is 200-400 bar, in particular 300 bar. Rail pressure increases of this kind are easy to effect and result in advantageous changes in the characteristics of the internal combustion engine. It should be noted, however, that the rail pressure increase and the differential rail pressure amount need not be constant, and the pressure values indicated are only exemplary. The rail pressure increase can be selected without restriction and can be optimized, for example, as a function of engine parameters such as engine speed, load, or even other parameters.
According to an exemplary method of the present invention, a maximum permissible injection volume per internal combustion engine stroke is established as a function of a turbocharger pressure of the internal combustion engine in the first, steady-state or quasi-steady-state load condition in accordance with a first characteristic curve, and in a second, non-steady-state load condition in accordance with a second characteristic curve, the maximum permissible injection volume in the case of a non-steady-state load condition being elevated in each case with respect to the maximum permissible injection volume in a steady-state or quasi-steady-state load condition, at identical boost pressure. This action, of increasing the maximum injection volume in the presence of a non-steady-state load increase, makes it possible to achieve optimum elevation of the engine torque in response to a non-steady-state load increase as a function of a boost pressure. In this context, the full engine torque is present only when the boost pressure exceeds a specific point on a smoke characteristics diagram curve and the injection volume, governed by engine speed, cannot be increased further. The combination of the elevation according to the exemplary method of the present invention of the rail pressure or injection pressure with the simultaneous increase in the limit quantity in the so-called smoke characteristics diagram may result, in particular, in the following advantages: the maximum exhaust gas turbidity (target value of the volume increase in the smoke characteristics diagram) remains the same, while the smoke pulse is shortened, and total particle emissions are reduced. Full engine torque may be reached sooner than with other approaches, and more torque may be available to the driver during the load increase. In actual driving, this yields advantages when accelerating and moving from rest. The overall efficiency of an engine controlled by way of the exemplary method according to the present invention is improved, resulting, for example, in lower specific consumption. The exemplary method according to the present invention can be implemented in software, and available EDC sensors may be used.
It is believed to be particularly advantageous that the injection difference is 15-25 mg, in particular 21 mg, per stroke. With differences on this order, the elapsed time until, for example, approximately 90% of the steady-state full load torque is available can be decreased by approximately 30% In addition, up to 20% more torque is available to the driver during a non-steady-state phase. The injection difference or elevation in injection volume that is selected also does not necessarily need to be constant; here again, the values indicated are exemplary. In this case as well, the limit values can be selected without restriction in the context of optimization.